This invention is in the field of gas chromatography and is specifically related to background correction means useful with plasma emission detectors for analyzing the output of a gas chromatograph (GC).
In gas chromatography a sample of interest is volatilized and injected into a gas chromatography column, typically housed in an oven. A carrier gas flows constantly through the column sweeping the sample along with it. Differential adsorption and desorption of the sample constituents on the partition medium in the column separates the sample into its components. Having been thus separated, the constituents of the sample elute from the column at different times and flow to a detector which continuously measures one or more properties of the gas eluting from the column. A change in the properties being measured relative to the baseline property of the carrier gas signifies that a sample constituent is passing through the detector. This is commonly referred to as a "peak". A recording of the detector signal, which may contain a large number of peaks, is called a chromatogram.
A variety of detectors are available to the chromatographer. The selection of what type of detector to use is a function of a variety of factors including the type(s) of samples being investigated, cost, sensitivity, selectivity and others. Some detectors respond well to a broad variety of sample species while others are useful for only specific types of compounds.
One type of detector which has gained increasingly widespread attention for use in gas chromatography is the plasma emission detector. In a plasma emission detector sample eluting from the GC column is introduced into a high temperature atmospheric pressure plasma where the sample molecules are broken up by action of the thermal energy into atomic or molecular fragments and ionized. As the species are swept through the plasma they undergo energy transitions and emit characteristic light spectra. Sample identification can be made by monitoring the wavelengths and intensities of the light so emitted. The plasma emission detector has been shown to be a highly sensitive universal detector. Microwave energy may be used to generate the plasma in this type of detector, which may then be called a microwave emission detector or "MED".
As a practical matter, a select discrete group of light frequencies is monitored in a plasma emission detector. (For purposes of this disclosure the term "light" should be understood to mean not only the visible portion of the electromagnetic spectrum, but also the infrared and ultraviolet.) These frequencies correspond to the characteristic emission wavelengths of certain sample species, generally elements, of the greatest interest. When a sample component of interest enters the plasma and is excited a signal is generated in the detector associated with the characteristic wavelength. Unfortunately, light at that characteristic frequency may be generated in the absence of the particular element or compound of interest thereby interfering with the analysis. This will be referred to as "background" radiation or light. One type of such background radiation will be a low level of general background radiation emitted from the plasma across a broad spectrum in the absence of any sample. In addition, other sample constituents in the plasma, particularly molecular fragments, may have complex emission spectra with lines (i.e., frequencies) near the line of interest. In gas chromatography of organic compounds this problem is particularly severe in the spectral region between 600 nm and 1000 nm where the formation of cyanide (CN) results in large increase in the background due to molecular band emissions in this region of the spectrum.
Prior art devices have utilized such techniques as increased resolution, wavelength modulation and locating a reference detector near the line of interest to correct for emissions not associated with the element of interest. Each of these techniques has its disadvantages however. Increasing the resolution, i.e., narrowing the frequency band which will generate a respond in the detector by using, for example, a narrow band filter, will help minimize interferences associated with nearby emission lines. But his occurs at the expense of absolute sensitivity for the line of interest. When the background noise is due to factors other than flicker in the light source, the noise will remain constant as the resolution is changed, so the effect of increasing resolution is to decrease sensitivity and, thus, signal-to-noise ratio.
In wavelength modulation, the signal to the detector is alternately moved between the intensity maximum and a point away from the maximum. Assuming that the underlying background signal remains at a constant level throughout the wavelength region of interest, the resulting signal has a constant contribution from the background and an amplitude modulated contribution due to the line emission. Using well-known techniques it is then easy to extract the modulated portion of the signal, thereby eliminating the background contribution. However, this techniques also results is decreased sensitivity and reduced signal-to-noise ratio.
In some instances, a reference detector is used to monitor changes in the background, and these changes are substracted from the signal at the wavelength of interest. However, this approach assumes that changes to the background at the reference detector are the same as the changes at the line of interest. This correlation often does not exist. In addition, depending on the physical construction of the detector apparatus, it may be difficult or impossible to locate a reference detector close enough to the detector for the wavelength of interest.
Accordingly, it is an object of this invention to compensate for changes to the background radiation at a wavelength of interests in a plasma emission detector used in a gas chromatography system to improve the quantitative and qualitative reliability of detection at said wavelength.
Another object of this invention is to provide a low-cost, first-order background correction method for use with a plasma emission detector connected to a gas chromatograph.
Yet another object of this invention is to provide a method of correction for background variations caused by the presence of carbon compounds in a sample under analysis.